The fundamental obstacle to effective gene therapy for hematopoietic diseases continues to be the difficulty in obtaining sufficiently high levels of gene transfer into Lympho-Hematopoietic Stem Cells (LHSC). At the core of this problem is the mismatch in the biology of the chosen target cell, i.e. the non-cycling LHSC, and the use of vector systems that preferentially transduce cycling cells. We hypothesize that a combination of vector modifications, alternative cell targets and selective expansion of corrected cells will be required to achieve adequate levels of gene correction for most diseases. Our goal in this proposal is to explore the potential of all three approaches. Our Specific Aims are: 1. To determine the relative capacity of Human Foamy Virus (HFV) and Human Immune-deficiency Virus-1 (HIV-1) based vectors to transduce quiescent human hematopoietic cells; 2. To determine whether lineage-committed progenitors, i.e. Common Lymphoid (CLP), and Common Myeloid Progenitors (CMP) offer advantages over pluripotent Lympho-hematopoietic Stem Cells as targets for gene therapy; 3. To determine whether expansion of transduced human LHSC and progenitors can be accomplished by conditional expression of a "cell growth switch" linked to a binding site for a chemical inducer of dimerization. This proposal brings together various complementary experimental tools, i.e. optimal vector systems for multi-lineage expression in primitive cells, the ability to isolate primitive human hematopoietic populations with defined lineage potential and the in vitro and in vivo systems to measure the lineage potential of transduced populations, perform clonal analysis and assess engraftment and in vivo expansion of each population. The biological interplay of the choice of vector, the type of target cells and the in vivo manipulation of the transduced target cells, will be analyzed in the context of the whole to understand the full therapeutic potential of these approaches.